1. Field of the Invention
The present invention relates to electric motor control circuits and is particularly concerned with a control circuit for split phase electric motors.
2. Description of the Prior Art
Split phase electric motors, as is known, comprise, in addition to the rotor winding, starting and running windings. The starting and running windings have respective impedances and when both are energized, a rotating field condition is simulated which causes the stationary rotor to commence rotating. When the rotor approaches the rated speed of the motor, which is determined by the number of poles in the stator, the starting winding is effectively deenergized by means of centrifugal or magnetic switch means or the like.
According to the present invention, and as disclosed in my prior U.S. Pat. Nos. 3,924,222 and 3,925,748, a positive temperature coefficient resistor element can be interposed in the starting winding circuit and the starting winding can be made substantially ineffective due to the increase in resistance of the resistor element referred to above when it becomes heated. The ratio of resistance of the resistor element between cold and heated conditions is substantial so that the current through the starting winding can be reduced to the point that no substantial loss of energy occurs due to the starting winding current.
It is quite often the case that motors of the nature referred to are protected by a temperature sensitive switch in series with the motor which is normally closed and which is actuated into open position under conditions of motor operation which would be damaging to the motor. An overload condition, or a condition in which the rotor was prevented from turning when the motor was energized, could lead to overheating of the motor and damaging of the windings thereof.
Under such circumstances, he current supply to the motor increases and a switch which is sensitive to excessive motor current can be provided in series with the motor and will respond to overload conditions and interrupt the supply of energy to the motor. In general, a switch of this nature can be made temperature sensitive by causing the motor current to pass through a bimetallic element or the like which, when raised to a certain temperature, will cause the switch to open.
Such a switch adequately protects the motor with which it is in circuit from damage due to overload and which positive temperature coefficient resistor referred to above forms an effective control for use in respect of cutting out the starting winding at the proper time, but conditions can arise in which the overload switch and the load responsive switch do not cooperate in the desired manner and the compromises which must be effected to make the switch arrangement work reliably detract from the efficiency thereof.
By way of example, if the resistor element is made of substantial size so as to have adequate current carrying capacity and to control the heating rate thereof, it is sometimes the case that this element will cool down only rather slowly. If it occurs that the overload switch were to open, then the possibility presents itself that the overload switch could close before the resistor element had cooled sufficiently to permit an adequate current supply to the starting winding. The overload switch, under such circumstances, could easily be triggered into open condition again. The described cycle could be repeated indefinitely without the motor starting under certain conditions.
An attempt to place the motor element and a temperature responsive overload switch in heat exchange relation proved to be unsatisfactory because ambient temperature could interfere with the operation, especially, of the overload switch.
The separating of the resistor element and the thermally operated overload switch resulted, as mentioned above, in compromising in order to obtain reliable operating conditions.
It has been discovered, as a result of tests and experimentation, that the problems heretofore encountered in respect of motor control circuitry of the aforementioned type can be made to operate in a highly satisfactory and reliable manner by the provision of a mechanical interlock between the high temperature coefficient resistor and the overload switch which prevents the overload switch from moving from an open to a closed position until the high temperature coefficient resistor element has cooled down to a predetermined temperature suitable for starting of the motor.
A primary object of the present invention is the provision of an improved motor control circuit and method especially adapted for use with a split phase electric motor.
Another object is the provision of a motor control circuit for a split phase motor having a high temperature coefficient resistor in the starting winding circuit and an overload switch in series with the motor with a mechanical link provided between the high temperature coefficient resistor and the overload switch.
A particular object of the present invention is the provision of a motor control circuit for a split phase motor having a high temperature coefficient resistor element in the starting phase circuit and an overload switch in series with the motor in which the overload switch after opening is prevented from closing until the positive temperature coefficient resistor element has cooled to a predetermined temperature.